2,6-Dimethylnaphthalene is a preferred feedstock for preparing 2,6-naphthalenedicarboxylic acid. 2,6-Naphthalenedicarboxylic acid is a useful monomer for the preparation of high performance polymeric materials. For example, 2,6-naphthalenedicarboxylic acid, or its methyl ester, can be reacted with ethylene glycol to prepare poly(ethylene-2,6-naphthalate) (PEN). Fibers and film manufactured from PEN have improved strength and superior thermal properties relative to other polyester materials. Films made from PEN demonstrate, for example, superior resistance to gas diffusion and particularly to the diffusion of carbon dioxide, oxygen and water vapor. Because of its exceptional properties, PEN is especially suitable for applications such as food and beverage containers, particularly for so-called "hot-fill" food and beverage containers, tire cord, magnetic recording tape and electronic components.
Although 2,6-naphthalenedicarboxylic acid can be prepared by a number of processes, perhaps the most preferred because of cost and efficiency, is the liquid phase oxidation of 2,6-dimethylnaphthalene. A suitable method for oxidizing 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid is described, for example, U.S. Pat. No. 5,183,933 to Harper et al. Although feedstocks other than 2,6-dimethylnaphthalene, for example 2,6-diethylnaphthalene or 2,6-diisopropylnaphthalene, can be oxidized to 2,6-naphthalenedicarboxylic acid, 2,6-dimethylnaphthalene is preferred because it is lower in molecular weight compared to 2,6-diethyl- or 2,6-diisopropylnaphthalene and, therefore, less 2,6-dimethylnaphthalene (by weight) is required to prepare a specified weight amount of 2,6-naphthalenedicarboxylic acid.
While 2,6-dimethylnaphthalene is present in certain refinery streams, for large scale use it is preferable to manufacture 2,6-dimethylnaphthalene starting with simple, readily available and inexpensive starting materials. One such process for manufacturing 2,6-dimethylnaphthalene is disclosed in Lillwitz et al., U.S. Pat. No. 5,198,594. The process for preparing 2,6-dimethylnaphthalene disclosed therein is called the "Alkenylation Process," which comprises reacting o-xylene with butadiene in the presence of a zero-valent alkali metal to form orthotolylpentene (OTP). The alkali metal-promoted reaction of an alkylaromatic with a conjugated diene such as butadiene to form an olefinically substituted aromatic is referred to as an alkenylation reaction. The OTP is subsequently cyclized to form 1,5-dimethyltetralin (1,5-DMT), the 1,5-DMT is dehydrogenated to 1,5-dimethylnaphthalene (1,5-DMN) and the 1,5-DMN is isomerized to the desired 2,6-dimethylnaphthalene (2,6-DMN). The overall process is summarized in equations (1) through (4) below. ##STR1##
In the alkenylation process for preparing 2,6-dimethylnaphthalene, the isomerization of 1,5-dimethylnaphthalene typically produces not only the desired 2,6-dimethylnaphthalene, but also a mixture of other hydrocarbon components such as 1,6- and 1,7-dimethylnaphthalenes and 1- and 2-monomethylnaphthalenes and various trimethylnaphthalenes. Additionally, other processes for preparing 2,6-dimethylnaphthalene and methods for isolating 2,6-dimethylnaphthalene from refinery streams generally require recovering the 2,6-dimethylnaphthalene from a mixture containing other hydrocarbons. In order to have a cost-effective process for preparing 2,6-dimethylnaphthalene, it is necessary to efficiently isolate the 2,6-dimethylnaphthalene from the mixture of hydrocarbons produced by the isomerization reaction, the hydrocarbon mixtures produced by other processes for preparing 2,6-dimethylnaphthalene, or mixtures produced during the processes for isolating 2,6-dimethylnaphthalene from refinery streams. The art, therefore, needs a simple and cost effective method for recovering valuable 2,6-dimethylnaphthalene from a mixture containing 2,6-dimethylnaphthalene and other hydrocarbon components. The present invention provides such a method.
Methods for isolating 2,6-dimethylnaphthalene from mixtures of hydrocarbons are known. For example, U.S. Pat. No. 3,806,552 to Oka et al., discloses that the separation of 2,6-dimethylnaphthalene from the isomerization reaction product can be easily carried out by cooling the isomerization reaction product to a proper temperature and separating the precipitated crystals, or by adding a suitable solvent to the isomerization reaction product, cooling the solution and separating the precipitated crystals. A number of suitable solvents are disclosed therein. U.S. Pat. No. 3,775,496 to Thompson et al., discloses that selective crystallization has been used to separate DMN (dimethylnaphthalenes) from each other, citing U.S. Pat. Nos. 3,485,885; 3,541,175; 3,590,091 and 3,594,436. U.S. Pat. No. 3,541,175 discloses a process for isolating 2,6-dimethylnaphthalene, and it discloses that crystallization may be carried out in a scraped cooling crystallizer. British Patent 1,345,479 discloses that 2,6-dimethylnaphthalene can be isolated by crystallization. As an example, 2,6-DMN is crystallized from a mixture containing 35.3 mol. % to 2,6-DMN, 36.8% 1,6-DMN, 8% 1,5-DMN, 7.7% 1,7-DMN and 4.2% low-boiling point and high-boiling point by-products.